LTC3731 datasheet(27/32 Pages) LINER

Part #	LTC3731

Description	3-Phase, 600kHz, Synchronous Buck Switching Regulator Controller

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Manufacturer	LINER [Linear Technology]

Direct Link	http://www.linear.com

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27

LTC3731

3731fa

PACKAGE DESCRIPTIO

PV

A

pF

VV

V

kHz

W

MAIN

IN

≈Ω

+

⎛

⎝

⎜

⎞

⎠

⎟

=

3

45

23

2

1000

1

51 8

1

18

400

6 3

2

()

()( )

()(

)

–.

.

()

.

This totals 1W at VIN = 8V, 2.25W at VIN = 12V and 6.25W

at VIN = 20V.

Total of all three synchronous MOSFET’s AC gate loss:

()

( )

( )(

)

(

)

36 15

4 5

Q

V

fnC

V

E

G

IN

DSSPEC

IN

DSSPEC

=

This totals 0.08W at VIN = 8V, 0.12W at VIN = 12V and

0.19W at VIN = 20V. The bottom MOSFET does not

experience the Miller capacitance dissipation issue that

the main switch does because the bottom switch turns on

when its drain is close to ground.

The Schottky rectifier loss assuming 50ns nonoverlap

time:

2 • 3(0.7V)(15A)(50ns)(4E5)

This totals 1.26W.

The total output power is (1.3V)(45A) = 58.5W and the

total input power is approximately 60W so the % loss of

each component is as follows:

Main switch’s AC loss (VIN = 12V) 2.25W 3.75%

Main switch’s DC loss

0.87W

1.5%

Synchronous switch AC loss

0.19W

0.3%

Synchronous switch DC loss

7.2W

12%

Power path loss

3.7W

6.1%

The numbers above represent the values at VIN = 12V. It

can be seen from this simple example that two things can

be done to improve efficiency: 1) Use two MOSFETs on the

synchronous side and 2) use a smaller MOSFET for the

main switch with smaller CMILLER to better balance the AC

loss with the DC loss. A smaller, less expensive MOSFET

can actually perform better in the task of the main switch.

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